This study aims to determine the distribution within the spinal cord of the nonmyelinated and small myelinated nerve fibers of the spinal dorsal root. These small fibers mediate pain and temperature sensation, while the larger myelinated dorsal root fibers mediate touch, muscle and position sense. Pain and temperature sensation is conveyed to the brain stem and thalamus by the spinothalamic tract. The study of the sites of termination of the primary afferents and their relation to the spinothalamic neurons is an essential step in defining the neuronal chain of the pain and temperature sensory pathway. At the root entry zone, the large fibers enter the dorsal columns, and also send collateral axons into the dorsal horn. The small fibers enter Lissauer's tract, sending branches into the dorsal horn. Two problems arise in tracing the small fibers within the dorsal horn: a) to distinguish the small afferent fibers from the axon collaterals of the larger afferents of the dorsal columns, and b) to distinguish the small afferent fibers from intrinsic axons of substantia gelatinosa neurons that also travel in Lissauer's tract and enter the dorsal horn. This study seeks to identify the small primary afferents by a combined use of autoradiography and degeneration methods. A high concentration of H3 amino acids is injected into a dorsal root ganglion of a rhesus monkey cervical cord. After allowing a few days for the distribution of the tritium label through both the large and small afferent axons, Lissauer's tract is selectively lesioned just rostral to the entering rootlets of the injected ganglion. After 2-5 days, the monkey is sacrificed. Using light and electron microscopy to examine the cord rostral to the lesion, these fiber types can be differentiated: a) collaterals of the large afferents, entering medial to Lissauer's tract, will be labelled but intact, b) small afferents from Lissauer's tract will be labelled but also degenerated, and c) small intrinsic fibers will be degenerated but unlabelled.